They were nude, doing all sort of erotic activity but she did not go for intercourse because she was committed to you! What an explanation!! I do not think this would have been possible as both of them nude and having other sexual activity. In any case she has not revealed this to you but you have found out on your own, that means she has cheated. I do not think you will be comfortable with her in future with this doubt running at the back of your mind.